Hello Peter.
[...]
Peter:
>>[big snip...]
>>>3) The homology of vertebrate limbs: the main problem with all
>>>similarities between functional features is that they may need to be
>>>similar in order to function: thus, they cannot be evidence for common
>>>descent.
Me:
>>I would agree with this if a singular component was being used for
>>comparison. For instance, there are examples of enzyme active sites where
>>strong arguments can be made for convergence rather than homology.
>>However, when the item being considered for homology is itself composed
>>of multiple components or features, as vertebrate limbs are, then I think
>>similarities of the component parts can be used to make a legitimate case
>>for common descent. I think that with more components available to produce
>>a "function" there would tend to be more ways to produce that functionality,
>>particularly in macroscopic systems such as limbs. That would tend to
>>undercut the argument for "similar functions requiring similar forms"
Peter:
>You are right. But there is one more problem: similar cladistic trees
>may be a consequence of the features concerned being interdependent.
Co-evolution.
>The function of limbs is certainly a consequence of the functions of many
>component parts. Now, if two components are functionally interdependent,
>their evolution is interdependent, and we would expect the two cladistic
>trees formed for the two features in a group of taxa to be similar, no
>matter how this came about.
Possibly, but...
What is the likelihood of two unrelated species using two unrelated
components to fulfill a similar function in a manner that renders
the components difficult to distinguish? I'd bet it can happen, but
relatively often, compared to the number of cases where the phylogeny
is clear-cut? Even in protein co-evolution there are often other parts
of the peptide chains which reveal information about ancestry.
>Just as a similarity between the same feature in two taxa may be a
>consequence of either common descent or common requirements, so a
>similarity between the cladistic trees constructed for two component
>features in the same group of taxa may be a consequence of either a
>descent acording to the cladistic tree found or of the two components
>being interdependent. This case has also been described in PSCF 44
>(2/1992), 80-94.
I'd bet this particular concern was discussed by taxonomists before 1992.
>Of course, if the compound function under consideration
>has been formed in different ways in different taxa, there will be at
>least some different interdependencies between the component functions
>or features, and there might be a stronger argument for evolution.
>
>>>Genuine evidence for homology may require functionless features (cf. my
>>>"How has life and its diversity been produced", PSCF 44 (2/1992), 80-94).
>> [...]
>>
>>That's definitely one strong class of evidence for homology. And I think
>>it's certainly a legitimate concern in molecular biology for studies about
>>the origin of single enzyme domains or protein families. But for multi-
>>enzyme or higher level, multi-component systems that are shared across
>>different species, it can be harder to make the case for convergence due
>>to functional necessity, particularly if the ultimate functions are not
>>quite the same across species (i.e. Consider the many end-functions of
>>vertebrate limbs in various species: terrestrial locomotion (quad- and
>>bipedal movement), aquatic propulsion, grasping, ripping, mating, cleaning,
>>burrowing, communication, & etc.)
>
>My argument given above includes the species-specific modifications of a
>given structure or function. If a given requirement is similar in two
>species, we expect the corresponding structures in the two species to be
>similar.
Grossly, perhaps. But not at the lower, structural levels. Both dolphins
and many fish have dorsal fins which they use for identical purposes.
However, these fins are structurally very different. All things being
equal (or undecidable, perhaps), I can see no a priori reason to suppose
that similar functional requirements will necessitate the same solutions
or the same routes. Only if the requirements severely limited the possible
solutions, or if historical factors (e.g. common ancestry) or horizontal
transfer opened the path to similar solutions would I expect otherwise.
>A natural corollary of this is that the differences between
>these requirements in the two species will lead us to expect
>corresponding differences between the two structures, no matter how
>this state of affairs came into being.
If dolphins and humans were separately created, why would they appear
to share many skeletal features, particularly in their fins/arms which
have very different functions? I think historical contingency is being
seriously overlooked as a major source of biological similarity. Sure, the
gross, external morphology of human and dolphin limbs are very different,
but the component structural elements reveal their common ancestry.
>We would expect limb buds to be very similar between frogs, ducks, dogs,
>and dolphins, but the further they advance in their embryonic development,
>the more they will differ from each other in order to function properly.
Possibly, but I'm not convinced that there wouldn't be exceptions.
The timing and location of developmental events may vary.
>The same would apply to any other multi-component system.
I would think that an increase in the available degrees of freedom in
a system would correlate with an increase in the number of possible,
non-identical solutions. Thus I'd be surprised if many multi-
component systems with separate evolutionary histories would converge
on forms that would be mistaken for having common ancestry.
Regards,
Tim Ikeda (tikeda@sprintmail.com)
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